Our current focus is on structural analysis of two simple three-component molecular complexes formed in transcription activation by the E. coil catabolite activator protein (CAP), RNA polymerase (RNAP), and promoter DNA. Structural knowledge of both CAP and RNAP and their individual complexes with promoter DNA elements is quite advanced, with particularly rapid progress made in the past few years. In contrast, knowledge about about the interactions between CAP and RNAP on promoter DNA in the two simple classes of transcription activation is still based largely on biochemical and genetic data. The goal of the proposed work is to advance structural knowledge of these interactions to yield complete, accurate molecular pictures of intact transcription complexes. The immediate aims are to obtain high-resolution structures of sub-assemblies of Class I and Class II CAP-RNAP-promoter DNA complexes using X-ray crystallography, and low resolution structures of intact Class I and Class II CAP-RNAP-promoter DNA complexes using cryo-electron microscopy. The work will yield complete, accurate molecular descriptions of intact transcription initiation complexes for the two major simple classes of CAP-dependent promoters, including descriptions of protein-protein interactions, protein-DNA interactions, promoter DNA structure, and role of solvent. Transcription activation by CAP is substantially simpler than most examples of transcription activation in bacteria and eukaryotes, which can involve complex arrangements of macromolecular components and/or DNA binding sites. The proposed studies of the CAP system provide a means to rapidly advance our understanding of fundamental structural principles underlying transcriptional control.